1. Technical Field
The present invention relates generally to determining appropriate aircraft safety clearance altitudes during the final approach of the aircraft to a runway, and more particularly to determining safety clearance altitudes for an aircraft between a Final Approach Fix (“FAF”) point and one or more step-down points located between the FAF and the runway.
2. Background Art
Systems are known in the art that provide alerts and advisory indications of premature descent. Among such systems are systems that monitor parameters of the aircraft's flight, particularly the aircraft's current position and flight path information, and a terrain elevation database.
Satellite-based navigational systems, such as Global Positioning System (“GPS”), which can track longitude, latitude, altitude, groundtrack, vertical velocity and ground speed, are becoming an important and reliable source of information for aircraft. An aircraft's Forward Looking Terrain Avoidance (“FLTA”) system compares aircraft elevation with terrain data in the region ahead of the aircraft during flight along and below the aircraft's lateral and vertical flight path to provide suitable terrain alerts if a potential threat exists of the aircraft colliding or coming too close to terrain. The computation involves searching through a terrain elevation database for terrain within the aircraft's potential flight path that may violate the aircraft's Required Terrain Clearance (“RTC”). The RTC is the value set by the Federal Aviation Administration as the permitted flight altitude for various phases of aircraft flight. The RTC indicates the clearance distance from terrain below which the aircraft should not fly. A Terrain Awareness Warning System (“TAWS”) is a system commonly used for commercial aircraft to specifically alert of potential terrain concerns. The Federal Aviation Administration (“FAA”) has instituted TAWS equipment standards that it publishes as Technical Standard Orders (“TSO”) (see e.g. TSO-C151b issued in December, 2002).
TAWS have been developed that utilize the advantages of GPS to evaluate the proximity of the aircraft to an airport and the flight altitude of the aircraft above a landing runway to determine if the aircraft is entering a landing procedure. For example, if an aircraft approaches the runway within a predetermined distance range and within a predetermined altitude range, the TAWS will determine that the aircraft is entering a landing procedure. During the landing procedure, the TAWS creates a terrain elevation boundary or minimum alert altitude surrounding the runway. An example of a system describing and explaining the use of a terrain elevation boundary and tracking of aircraft position using a Global Positioning System (“GPS”) may be found in U.S. Pat. No. 5,839,080, entitled “Terrain Awareness System.” General use of a terrain elevation boundary for calculating and providing terrain alerts during both cruising and landing procedures is well know in the art. By adjusting or desensitizing the aircraft terrain clearance values during a landing procedure from the minimum clearance values required during aircraft cruising flight, nuisance alerts may be reduced.
To provide higher levels of safety during landing yet reduce nuisance alerts, accurate methods of identifying when landing procedures are initiated and accurately identifying an appropriate destination runway is desirable. U.S. Pat. No. 6,304,800, entitled “Methods, Apparatus and Computer Program Products for Automated Runway Selection” discloses a method of identifying a destination runway.
Aircraft using TAWS also provide Premature Descent Alerts (“PDA”) during the final approach of the aircraft to the runway to indicate whether the aircraft is descending below a “normal” final approach flight path to the nearest runway (typically a 3 degree implied glideslope). Premature Descent Alerts are generally evaluated and produced independent of in-flight terrain alerts.
A typical TAWS may further include a fixed, standard terrain clearance level for all landing approaches and runways. If the aircraft is below the standard terrain clearance altitude for a given radial distance from the runway, an alert is provided. One example of a conventional standard for determining landing clearance altitudes is a system that for all runways and approaches establishes a landing clearance altitude (vertical distance above the runway elevation) at 700 ft clearance for greater than 15 nautical miles (nm) from the runway, slope down to 350 ft for a range between 5 and 12 nm from the runway, slope down to 150 ft for a range between 1 and 2 nm from the runway, and slope down to zero ft from ⅓ nm to the runway, resulting in a step-down approach to the runway. Another standard fixed terrain clearance system uses a 300 ft clearance for between 5 and 2.5 nm that begins sloping from 300 ft to zero at 1 nm to the runway. Another similar approach does not establish vertical altitude levels, but rather establishes fixed terrain clearance levels for fixed radial distances from the runway. In either case, however, a flight safety altitude is established below which the aircraft should not travel. Please reference FIG. 1 for a visual representation of this example.
Conventional Premature Descent Alert systems that provide fixed altitude flight levels or fixed terrain clearance levels for fixed radial distances from the runway regardless of the runway or approach to the runway are insufficient in that they are not conducive to every runway and approach. One particular example that may cause particular problems for a fixed altitude flight level system is where a particular runway is nestled in a valley or has a large tower near it. Ordinarily, for this situation, the pilot would need to approach at a higher altitude and make a more rapid descent than for other runways. With a fixed altitude alert system, the pilot may descend to a level below the height of the mountain or tower prior to reaching it and be required to again ascend to clear the obstacle. The Premature Descent Alert system was programmed only to provide alerts if the aircraft descended below the fixed altitude regardless of the runway. Accordingly, only the related terrain alert systems were used to identify the obstacle. In some systems, however, the terrain alert systems are desensitized to reduce nuisance alerts during landing. This may delay crucial safety warnings and alerts that could otherwise be provided. For a different approach to the same runway, the circumstances of the approach may have been different with different obstacles and terrain.
Another particular example that may cause problems for a fixed clearance flight level system is where the runway is raised significantly above the surrounding terrain such as on a mountain, a mesa, or a cliff. While the fixed clearance levels referenced may have allowed the aircraft to receive necessary alerts in relation to the previous example, the fixed clearance levels may have problems with this example. In this case, the aircraft may have 150–300 feet of clearance above the surrounding terrain and still be below the runway elevation. Again, the pilot will need to correct the flight path, but will only become aware of the needed correction when the pilot gets much closer to the runway. In this case also, if the terrain alert systems are desensitized, crucial safety warnings and alerts may be further delayed until a smooth correction is no longer possible.
Thus, many conventional systems produce landing altitude boundary values that have a fixed radial distance to altitude relationship for every case. In other words, each specific radial distance from the runway always has a fixed associated altitude boundary or flight safety altitude associated with it regardless of the approach direction and regardless of the runway. As a result, conventional systems do not compensate for actual aircraft approach flight paths, runway environment, or airport obstacles, but rather focus only on flight safety altitudes for given radial distances from the runway. This may result in excessive nuisance alerts or added risk of collision with terrain or a structure during final approach because the flight safety altitudes selected as the general rule may not be conducive to a particular approach to a specific runway.